The effects of foliar applied urea and potassium chloride on the severity of leaf diseases of cereals were investigated in the laboratory, glasshouse and field between 1992 and 1995. Field studies with urea gave inconsistent results with respect to severity of Erysiphe graminis and consistently increased the leaf area affected by Septaria tritici. However, potassium chloride applied as a foliar spray consistently decreased the leaf area of wheat affected by E. graminis and S. trifid compared with equivalent applications of soil applied fertiliser. Disease control was achieved at early stem extension and after flag leaf emergence but yield responses were not detected. Laboratory investigations were undertaken to determine the mechanism by which foliar applied potassium chloride reduced the leaf area affected by E. graminis. The timing of application, within seven days pre or post inoculation, had no consistent effect on the efficacy of the fertiliser. Investigations using polyethylene glycol as a control showed that the percentage leaf area affected declined linearly as the osmotic potential of the solutions were increased. Light microscopy revealed that the germination of spores in solution and on treated leaves was reduced as the osmotic potential of the solutions were increased. Spores which did germinate developed normally but those on leaves treated with solutions of high osmotic potential rarely formed haustoria. This suggested a second mechanism acting inside the leaf. Multiple regression analysis of experimental data indicated that the inhibition of spore germination was the major response reducing the area of the leaf affected. Although the data were not conclusive it appeared that the increase in leaf water potential, following the foliar application of potassium chloride, was involved in the control of E. graminis.